Is this a tachometer?



I will look a little closer. I thought that the wires were too think to carry the main motor current. I will try to find out for sure tonight.
i
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Did you resolve it?
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
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Wes, yes, I am 100% certain that it is a tachometer. 9.5 volts per 1000 RPM.
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Interesting. The 4 brushes seem like overkill for a tach. I wonder why a two brush design wasn't good enough. EE's, got an explanation?
Wes
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Wes wrote:

Yup, the big problem with DC tachs is keeping the brushes in good contact with the commutator. My guess is they use an odd number of slots/windings/commutator segments. With only two brushes, you'd still have one brush jumping across commutator segments every small amount of rotation. With 4 brushes, paired with their opposite brush as you do with a 4-pole DC motor, then there is always one of the pair making contact with the commutator. So, this should greatly reduce brush noise and produce a much cleaner tach signal. Some small tachs use a silver-plated commutator and sliver-wire brushes. These have limited life, however, like the Maxon motor/tachs.
Jon
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That makes a lot of sense to me. Thank you for the explanation.
Wes
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Wes wrote:

Lower contact noise from the commutator. This allows for a higher degree of accuracy in the control loop.
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Michael A. Terrell wrote:

Some years back a mate of mine mentioned he was involved in an instrumentation project on a diesel engine in a bus or truck and they used a synchronous motor, as found in mains timers etc, for the tachometer. IIRC the AC generated was rectified but I can't recall what they did with it after that, maybe a frequency to voltage converter. Do you have any ideas as to the benefits or disadvantages of this sort of tachometer. I can see that there would be no contacts so possibly reduced noise but not sure what effect methods of converting the AC to a DC voltage proportional to RPM would have in the system. I have put a synchronous motor onto a scope and spun it to generate a AC waveform but not tried it beyond that.
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David Billington wrote:

You could use either a frequency to voltage converter, or a simple counter circuit. The F/V converter is a simple, single chip design, and cheap. The counter gives the option for accurate speed warnings or overspeed shutdown. Cost is probably five times the F/V converter and a meter movement.
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    [ ... ]

    The DC tach provides pretty much a constant voltage for a given speed -- even at a near standstill. I would expect the frequency of the generated AC to be so low that even rectified and filtered, you would get variation in the output voltage for a full rotation with a constant speed.
    And with rectified and filtered AC output, you would have much slower response to variations in actual motor speed, so the ability to control the speed would suffer.
    I believe that the AC servo motors have a pair of Hall effect sensors (at 90 degree spacing) and a rotating permanent magnet, and analyze the two signals to determine the actual physical position, and from rate of change in that determine the speed. A lot more electronics to do the same thing -- but the advantage is that there are no brushes to wear out.
    Enjoy,         DoN.
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wrote:

typically not an AC generator but a selsyn for this application (did I spell that right? special variant of a synchro resolver)
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    My guess is a slight overlap of switching time between two sets of brushes so one switches while the other makes full contact to minimize glitches in the feedback signal -- which could cause glitches in the speed of the motor, and mar the finish of the workpiece.
    Enjoy,         DoN.
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